The present invention relates generally to the field of distributed power generation, and more particularly to an improved load follower configuration of a distributed power generation system.
Distributed power generation concerns the movement of power generation devices from central, large scale power plants to smaller units located close to the end-users, thereby enabling significant environmental benefits to be derived from the use of sustainable energy sources. However, various forms of distributed generation are unable to adjust their output power fast enough to follow fast-changing loads. Fuel cells and microturbines are two such examples. Other types of distributed generators such as photovoltaic and wind-turbines produce fluctuating output power as a function of available energy (e.g.xe2x80x94sunlight or wind). Both slow-load-following generators and fluctuating-power generators are typically paired with some type of energy storage to operate as a stand-alone generation system.
The most common type of energy storage paired with distributed generators in stand-alone systems is a chemical batteryxe2x80x94typically lead-acid battery. During operation of each type of system, the battery has to provide power for instantaneous load changes. For example, in a fuel cell system, the fuel source and the system operating temperature cannot be quickly adjusted. While these and other parameters are adjusted to match the required load, a battery either provides power to or absorbs power from the load, depending on whether the load is increasing or decreasing. In a stand-alone photovoltaic system, the photovoltaic arrays typically charge a bank of batteries and the batteries directly provide power to the load. In all of these systems, the rate of battery charge and discharge can vary considerably with time and is subject to the variability of the load. The batteries can therefore experience frequent cycling, and the level of discharge in that cycling can be shallow or deep, depending on the load and the availability of generator power.
FIG. 1 shows a typical power conversion system for a distributed generator system that includes a battery that is used for load-following. The system includes a generator 10, battery 12, and first and second power converters 14A and 14B (Power Converter1 and Power Converter2, respectively). The generator 10 provides power to the first power converter 14A. The output voltage of the generator may be dc, low-frequency ac, or high-frequency ac, depending on the output of the generator. The first power converter 14A is designed to convert the generator voltage to a relatively constant voltage on a dc link. This voltage may then be further processed if necessary to provide the correct voltage to the load. In some applications, for example, telecom power supplies, the dc link may be connected directly to the load.
The battery 12 is able to provide power to or draw power from the dc link through the second power converter 14B. In some cases, power converter 14B may be replaced by a direct connection of the battery to the dc link. In that case, adjusting the voltage of the dc link could control the charging and discharging of the battery 12. In other cases, the second power converter 14B can directly control the charge drawn from or the charge inserted into the battery.
When there is a sudden load increase, the second power converter 14B will draw power from the battery 12 equal to that increase until the generator 10 is able to respond. When the load is steady or decreasing, the generator can provide some additional power to charge the battery.
Most types of batteries exhibit shortened lifetimes when subjected to frequent cycling. Furthermore, some types of batteries exhibit a phenomenon often referred to as xe2x80x9cmemoryxe2x80x9d. A battery that exhibits a xe2x80x9cmemoryxe2x80x9d phenomenon xe2x80x9cremembersxe2x80x9d its previous cycling history. For example, if a NiCad battery is only partially charged before being discharged, it cannot be fully charged again. This process worsens with partial charge cycles. A NiMH battery exhibits similar phenomenonxe2x80x94it will not provide full power at low charge levels if it was previously discharged only partially before recharging. The worsened performance in NiMH batteries is reversible whereas the worsened performance in NiCad batteries is not.
As a result of the xe2x80x9cmemoryxe2x80x9d effect, many types of batteries are not used in load-following applications. As a consequence, the most common type of battery for load-following is the lead-acid battery. However, lead-acid batteries have a number of shortcomings that make them undesirable for many load-following applications. Cycling them significantly shortens their service life. Furthermore, they contain lead and therefore offset one of the main benefits achieved by many forms of distributed generationxe2x80x94an environmentally safe solution to generation. Lead-acid batteries also have a relatively inefficient round-trip efficiency. Just keeping them charged can take a significant amount of energy. Since distributed generation is relatively expensive on a cost per kilowatt basis, a high efficiency load-following scheme is crucial to make the overall system cost-effective.
The present invention permits the use of batteries that exhibit a xe2x80x9cmemoryxe2x80x9d effect in load-following applications. The preferred embodiments use two batteries. At any given time, one battery is only discharged while the other battery is only charged. When one battery reaches full charge and the other reaches full discharge, the role of the two batteries is reversed. In this way, the memory effect does not reduce the performance of the batteries. Other aspects of the present invention are described below.